Generally, in manufacture of aluminum alloy heat exchangers, the brazing process using Al—Si-based alloy is carried out as a method of jointing the component aluminum members. Especially in the brazing process using aluminum alloy sheets as materials, a so-called aluminum alloy brazing sheet, which is an aluminum alloy composite material that is made into a sheet material by laying a filler alloy made of Al—Si-based alloy on the surface of a core material made of aluminum alloy, is used. For example, a brazing sheet made of aluminum alloy core material clad with Al—Si-based filler alloy on its both sides is used for a coolant path composing a core of an automobile evaporator.
FIG. 1 is a perspective view of an example of a basic structure of the assembled state of a tube fin portion of the evaporator core having an inner fin. FIG. 1 shows the inside of the core in a section. As shown in the figure, a tube material 11 in which a coolant path 16 is set up and formed, and a corrugated inner fin material 13 is installed in the coolant path 16, and an outer fin material 12 for which aluminum alloy sheet material is corrugated and installed on the outside of the core, are assembled alternately one on top of another. Reference number 14 in the figure is the inner joint portion of the tube material 11, and 15 shows the joint portion where the corrugated inner fin material 13 installed in the coolant path 16 is joined with the tube material 11.
In FIG. 1, the side to which the inner joint portion 15 where the inner fin material 13 and the tube material 11 are joined belongs, or the side to which the tube joint portion 14 belongs, is called an inside of the hollow structure, and the side where the outer fin material 12 and the tube material 11 are joined is called an opening portion. Likewise, in the structure of the drawn cap shown in FIG. 7, the side of the inner joint portion 74 of the drawn cap is called the inside of the hollow structure, and the outer joint portion 75 of the drawn cap is called the opening portion.
In order to braze brazing sheets, it is necessary to destroy the hard oxide film generated on the surface of the filler alloy during braze heating. The brazing method can be classified largely into two: the vacuum brazing method conducted in high vacuum and the NB (nocolok brazing) method using flux.
Of these two methods, the vacuum brazing method conducted in high vacuum is the method of brazing without using flux. In this method, it is indispensable to contain about 1.5% by mass of Mg in the filler alloy of the brazing sheet. Mg destroys the hard oxide film of the filler alloy when it evaporates from the brazing sheet by braze heating. Further the evaporated Mg plays the role as getter that removes oxygen and moisture remaining in the furnace. As a result, brazing is made possible by the vacuum brazing method conducted in a high vacuum.
This vacuum brazing method enables brazing even for a very complicated structure, but the problem is that costly large-scale equipment must be used to control the atmosphere. Another problem is that the effect of sacrificial corrosion prevention due to Zn cannot be obtained because the Zn that was added in the material evaporates in the vacuum brazing method.
Meanwhile, the brazing method in an inert gas atmospheres using noncorrosive flux (hereinafter to be referred to as NB method), which is currently the mainstream method, makes brazing possible as the flux destroys the oxide film of the filler alloy. In this method, braze heating is conducted in an inert gas atmosphere, and flux coating on the surface of the filler alloy is indispensable.
Moreover, since this method uses noncorrosive flux for in-furnace brazing in an inert gas atmosphere, it is easy to control the oxygen concentration of the furnace atmosphere by using an inert gas. Accordingly brazing can be easily conducted on an industrial scale. Further, since noncorrosive flux is used, it is not necessary to remove the flux after braze heating. Thus, this method is most widely used especially in the manufacture of heat exchangers for automobile.
In the aluminum alloy brazing by the NB method, flux is indispensable to remove the oxide film formed on the surface of the aluminum alloy, and flux must be touched to the joint portion before melting the filler alloy. However, in the heat exchanger with the structure as shown in FIG. 1, if flux is applied after assembling the tube material and the inner fin material, brazing could be defective because flux does not spread sufficiently in the tube. Therefore, flux must be applied at a stage of material or press-molded parts before assembled to the core. In this procedure, handling tube materials coated with flux is bothersome, and defective brazing easily occurs as flux can not evenly spread to necessary portions, each of which are not desirable for industrial purposes.
Further, since the residue of flux remains in the tube of the heat exchanger core brazed in this manner, the tube is sometimes clogged with the residue of flux, which could cause performance degradation of the heat exchanger.
In order to solve the above problems, a method of brazing the inside of the tube without applying flux has been proposed. In this method, brazing is conducted without flux in an inert gas atmosphere of atmospheric pressure, using a filler alloy that contains 0.2% to 1.5% by mass of Mg further in the Al—Si series filler alloy generally used in the NB method (Specification of U.S. Pat. No. 5,839,646). Accordingly, it is possible to braze the inside of a hollow structure such as a tube without coating with flux. However in this method, minute amounts of oxygen existing in the furnace atmosphere reacts with Mg contained in the filler alloy during braze heating and forms Mg oxide film on the surface of the filler alloy, so it is necessary to reduce the oxygen concentration in the furnace atmosphere. In the conventional NB method, brazing is possible by making the oxygen concentration of the furnace atmosphere 200 ppm or less. In contrast, in the method in which Mg is contained in the filler alloy, the oxygen concentration of the brazing furnace atmosphere must be about 10 ppm or less and thus this method is not industrially practical. Moreover, since brazing must be done in strictly regulated brazing atmosphere to an oxygen concentration of about 10 ppm or less, unstable brazing performance occurs in the portion where complete atmosphere substitution is difficult such as the deep inside of the tube. Further especially a portion such as around the opening of the tube, which is exposed to the flow of the furnace atmosphere, is easy to be affected by the oxidizing atmosphere, and deposition of MgO is remarkable there. Alternatively the flux atmosphere coming from outside and the Mg in the filler alloy react each other. This makes problems of the impossibility of brazing in the portion such as around the tube opening easily affected by the furnace atmosphere.
In another brazing method, the whole members to be brazed are put inside a housing, in which, for example, Mg is placed, to enable no-flux brazing in an inert gas atmosphere. (Japanese Patent Laid-Open No. JP-A-9-85433). In this method, brazing is conducted on the condition of covering a member to be brazed with a Mg-added material or placing pure Mg inside of the housing. In yet another method, brazing heat exchanger members placed in a housing is conducted, where Mg is added to the filler alloy of the member to be brazed or to the structural material of the member to be brazed.
In the method using the Mg in the housing or using the Mg placed inside the housing, evaporation of Mg hardly occurs due to brazing under atmospheric pressure, and surely brazing is difficult since the amount of Mg reaching the portion to be brazed is extremely small. On the other hand, in the method in which brazing the heat exchanger members placed in the housing is conducted, where use is made of a material in which Mg is added to the filler alloy of the item to be brazed or to the structural material of the item to be brazed, the member to be brazed must be placed inside of the housing every time of brazing, and this is an inefficient method for industrial production.
Further, besides the above brazing methods, there is another method of inert gas atmosphere brazing called VAW method in which flux is not used. In this method, brazing is enabled in an inert gas atmosphere by adding minute amounts of Bi, Sb, Ba, Sr, Be, etc. to filler alloys and destroying and removing the oxide film on the surface of the filler alloy by means of alkali etching or acid etching before braze heating. However in this method, the atmosphere must be strictly controlled to a dew point of −65° C. or less and an oxygen concentration of 5 ppm or less. Moreover, pretreatment of material is necessary and strict control of the atmosphere is necessary, so this method is not suitable in terms of practical use.
Recently, as slimming down of material progresses, the thickness of filler alloy is also decreasing. Since the decrease in quantity of filler alloy to be used tends to degrade the brazing quality, it has become necessary to secure a good brazing quality even in the case of using thin filler alloy.
Thus, still there remain problems to solve in brazing for industrial manufacture of heat exchangers.
It is an object of the present invention to provide an industrially efficient method of brazing aluminum or aluminum alloy materials and also a brazing sheet suitable for this method, whereby the above-mentioned existing problems are solved and brazing can be easily conducted without coating with flux in the tube where applying flux is difficult in the NB method.
It is another object of the present invention to provide a brazing sheet that has a low occurrence rate of a break in a brazing portion (defect owing to insufficient supply of filler alloy) even in the case of a thin clad filler alloy, in the method that enables no-flux brazing of the inside of a hollow structure, by using a brazing sheet containing Mg in the component member.
Other and further objects, features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.